The present invention relates generally to communication systems, and more particularly to time diversity techniques for use in improving signal-to-noise ratio (SNR) or other performance measures in a communication system.
In packet telephony or broadcasting applications, such as voice transmission over asynchronous transfer mode (ATM) or Internet protocol (IP) networks, channel impairments often result in conditions in which transmitted information packets are dropped due to congestion or interference. A well-known conventional technique for addressing this problem involves repetitive transmission. In repetitive transmission, the same block of signal information is quantized, encoded and transmitted several times, e.g., N times as N separate packets containing the same information. As an example, consider a signal       [          S      ⁢              xe2x80x83            ⁢              (        i        )              ]    =            S      ⁢              xe2x80x83            ⁢              (        i        )              |                                        L                                                              i              =              1                                          .      
The quantized and reconstructed version of this signal is given by       [                  S        _            ⁢              xe2x80x83            ⁢              (        i        )              ]    =                    S        _            ⁢              xe2x80x83            ⁢              (        i        )              |                                        L                                                              i              =              1                                          .      
It can be shown that
xe2x80x83S(i)={overscore (S)}(i)xe2x88x92n(i) or {overscore (S)}(i)=S(i)+n(i)
where n(i) is the quantization noise, assumed for this example to be independent, identically distributed (iid), and independent of {overscore (S)}. The signal-to-noise ratio (SNR) for this repetitive transmission technique is       ⟨          S      2        ⟩        ⟨          n      2        ⟩  
regardless of the number of repetitions. When packet loss occurs, the signal can still be recovered as long as any one of the N packets is received by the receiver. The signal blocks are processed sequentially without overlap.
The above-described repetitive transmission technique does not attempt to improve SNR performance, but is instead directed only to alleviating the problem of packet loss. The technique is inefficient in terms of coding gain given that it consumes N times the bits otherwise required to transmit the same information. A need thus exists for signal transmission techniques which can provide improved SNR performance, as well as robust signal reconstruction under adverse transmission conditions.
The invention provides improved repetitive transmission techniques which utilize time diversity to increase the signal-to-noise ratio (SNR) of a reconstructed composite signal in the presence of packet loss. In accordance with the invention, time shifts are introduced between N versions of a particular block of information to be transmitted, and the time-shifted multiple versions are encoded in a set of N encoders and transmitted as N packets. The time shift introduced between a given pair of the N versions corresponds to approximately 1/N of the time duration of a particular one of the versions. The SNR of a composite reconstructed signal generated from the N packets with the introduced time shift in a receiver of the system is approximately the same as would be obtained using a set of N independent encoders to generate the plurality of packets without the introduced time shifts. The gain in the SNR of the composite reconstructed signal attributable to the introduction of the time shifts is 10 log10Nxe2x80x2, where Nxe2x80x2=1, . . . N is the total number of the N packets actually received at the system receiver.
In accordance with another aspect of the invention, a further improvement in SNR performance may be obtained by introducing quantization error compensation, in which quantization error from the encoding of a given one of the versions is successively combined with subsequent versions prior to encoding of those versions. In this case, the gain in the SNR of the composite reconstructed signal attributable to the introduction of the time shift and the quantization error compensation is
20 log10Nxe2x88x9210 log10[2(Nxe2x88x92Nxe2x80x2)+1],
where, as noted above, Nxe2x80x2=1, . . . N is the total number of the N packets actually received at the system receiver.
In accordance with other aspects of the invention, each of the N packets may be generated by a corresponding one of N independent encoders, such that their quantization noise components are independent; and the particular value of N may be made adaptive to network conditions such as packet loss rate, e.g., N may be made large if the packet loss rate is high.
Advantageously, the invention provides substantially improved SNR performance relative to conventional repetitive transmission, without significantly increasing transmission complexity. Although particularly well-suited for use in packet voice transmission applications, the invention can be applied to the transmission of any other type of digital information, including, e.g., data, audio, video and image information. In addition, the invention may be implemented in a wide variety of packet-based communication systems, such as, e.g., ATM or IP networks, Internet and satellite broadcasting systems, digital audio broadcasting systems, etc.